The known process and apparatus for making hot-rolled steel strip from a striplike continuously cast starting material use successive processing steps in which the striplike continuously cast starting material after solidification is brought to the hot rolling temperature and fed to a multi-stand rolling mill for rolling to the finished rolled product.
Of course attempts have already been made to further roll continuously cast starting material issuing continuously from a continuous casting unit. The principal difficulty with this arises because the maximum casting speed with which the casting leaves the continuous casting unit is much less than the lowest possible rolling speed of a conventional line of rolls comprising the multi-stand rolling mill which for example can include seven roll stands.
The striplike starting material used to form the casting generally has a thickness in the range between 25 to 60 mm. If one starts with a central strip thickness of about 40 mm produced by a casting speed of about 0.13 m/s and presumes that the strip should be rolled to a thickness of 2 mm, there must be a twenty fold change.
In continuous operation under the supposition that the casting speed is equal to the inlet speed in the first roll stand in a tandem line with seven roll stands, the resulting outlet speed at the last roll stand is about 2.67 m/s.
The minimum outlet speed with a rolled product thickness of 2 mm amounts to about 10 m/s however, since at the lower speed an excessive temperature decrease makes the rolling impossible.
This problem could be dealt with now in two ways. In one approach a multi-stand continuous rolling mill or line of rolls is replaced by a powerful shaping unit (e.g. a planet rolling mill) which operates with a reduced entrance speed at the roll stand and with which a high reduction per pass can be attained (see Berg- and Huttenmannische Monatshefte, Vol. 107. Jg., page 149).
However up to now no satisfactory results have been obtained even with very expensive special structures, the uniformity of the rolled stock was lacking.
A process known from German Open Patent Application No. DE-OS 32 41 745 proposes a solution. The striplike casting is rolled up into a roll or bundle and, after heating, is again unrolled and fed to a rolling mill for rolling to its final cross section. The rolling mill is then a pin or peg rolling mill or a finishing rolling mill group of a hot strip rolling mill.
Disadvantageously this known unit has a high investment cost for a multi-stand tandem line which may run over 40 million dollars. These high costs are only compensated when the konti line of rolls is completely balanced. Therefore as set forth in the named reference, the konti line is put in front of a multi-branch continuous casting unit. However because of that the total cost of the plant is increased in addition to the output capacity of the entire unit which in many applications is not at all necessary.